Packaged circuit boards for electronic instruments are typically coated by a moisture-proof insulative film to protect the circuit boards from moisture, electric leakage and dust. The moisture-proof insulative films are what are known as conformal coatings, such as acrylic, polyurethane or epoxy synthetic resins dissolved in a volatile solvent. When applied to a clean printed circuit board, an insulative resin film is formed as the solvent evaporates on a continuous basis.
One preferred method and apparatus for applying liquid coating material to printed circuit boards is disclosed in U.S. Pat. Nos. 4,753,819 and 4,880,663, both to Shimada, which are owned by the assignee of this invention. In accordance with the method disclosed in these patents, printed circuit boards are coated with an insulative coating of high quality, in terms of thickness, uniformity and the absence of splashing, by applying the liquid coating material to an airless flat pattern nozzle at a supply pressure, e.g., five Kg/cm.sup.2 for a 100 cps viscosity coating, such that a liquid film emission from the nozzle is produced in the form of a flat, leaf-shaped pattern. The leaf-shaped pattern, which is substantially free of atomized particles, has a maximum width at approximately its center and decreases in width is on both sides of the center to approximately zero width at the discharge outlet of the flat spray nozzle and at the end of the pattern. The nozzle and printed circuit board are spaced relative to each other to locate the circuit board in the path of the flat, leaf-shaped liquid film pattern where the width of the pattern is maximum. The method disclosed in the Shimada patents is therefore predicated upon the concept of operating airless spray nozzles at pressures substantially below the pressures at which they are normally operated, i.e., atomizing pressures. One airless, flat spray nozzle of the type which has been used for this purpose is disclosed in U.S. Pat. No. 4,349,947, owned by the assignee of this invention.
Despite obtaining a splash-free pattern of coating material on the printed circuit boards, the nozzles employed in the method and apparatus disclosed in the Shimada patents have certain limitations. One problem with such nozzles has been that the width of the leaf-shaped pattern, even at its maximum point, is relatively narrow. As a result, a relatively long period of time is required to complete coating of a printed circuit board because the nozzle must be moved in a series of side-by-side rows and/or columns a sufficient number of times to cover the entire surface area of the circuit board. Additionally, it has been found that such airless spray nozzles produce a flat spray or "ribbon" pattern which tends to be uneven across the width of the pattern. Moreover, if the pressure at which the coating material flows through the nozzle is increased or decreased, the thickness of the film or layer of coating material applied to the printed circuit board can also vary.
A still further limitation of airless spray nozzles of the type employed in the Shimada patents is that the leaf-shaped pattern emitted therefrom has a maximum width at a relatively short distance from the discharge outlet of the nozzle. As mentioned above, it is desirable to position the nozzle with respect to the printed circuit board such that the leaf-shaped pattern is at maximum width when it contacts the board in order to obtain maximum coverage of coating material. Because this point of maximum width of the leaf-shaped pattern is relatively close to the discharge outlet of prior art spray nozzles, sufficient clearance may not be provided between the nozzle and some of the elements on the printed circuit board without raising the nozzle vertically upwardly relative to the board by robot manipulators or other gun moving devices. Movement of the nozzle upwardly relative to the printed circuit board complicates the coating operation and can reduce the width of the pattern obtained on the board because the tapered leaf-shaped pattern decreases in width in both directions from its centerpoint. This can produce gaps or spaces in the layer of coating material applied to the printed circuit board where the patterns are is applied side-by-side in columns or rows as the nozzle and printed circuit board are moved relative to one another.